multiple: set sane defaults

Set sane defaults for some types rather than setting them to `undefined`.
This is so that we can set `exactOptionalPropertyTypes` to true in our
tsconfig.

src/Components/CreateImageWizard/steps/Repositories/Repositories.tsx

@@ -133,7 +133,7 @@ const Repositories = () => {
       url:
         toggleSelected === 'toggle-group-selected'
           ? [...selected].join(',')
-          : undefined,
+          : '',
     },
     { refetchOnMountOrArgChange: 60 }
   );